"It has been well observed that phytoplankton can control their
movements in the water and move toward light and nutrients,"
marine scientist Susanne Menden-Deuer, of the University of Rhode
Island, said in a statement. "What hasn't been known is that they
respond to predators by swimming away from them. We don't know of
any other plants that do this."

In lab experiments, Menden-Deuer and her team observed that
groups of the phytoplankton Heterosigma akashiwo flee
when in the presence of predatory zooplankton. What's more, the
tiny plants swim away from areas that previously contained the
predators even if the immediate threat is gone.

"The phytoplankton can clearly sense the predator is there,"
Menden-Deuer said. "They flee even from the chemical scent of the
predator but are most agitated when sensing a feeding predator."

The algae do this to stay alive. If the phytoplankton have no
place to hide, they'll get eaten up by their
zooplankton predators within a day. But the algae double in
population every two days if they have a refuge, the researchers
said. If the same is true for other species of phytoplankton,
this discovery could offer a new explanation for some
plankton blooms.

"One of the puzzling things about some phytoplankton blooms is
that they suddenly appear," Menden-Deuer said. "Growth and
nutrient availability don't always explain the formation of
blooms. Our observation of algal fleeing from predators is
another mechanism for how blooms could form. Amazingly, looking
at individual microscopic behaviors can help to explain a
macroscopic phenomenon."